Art of Codeartofcode
Lesson 16 of 17

38. Design a Chat App (WhatsApp)

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The interview prompt
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38.1 What do you ask first?
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38.2 How big is this, really?
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38.3 What are the doors?
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38.4 What do we store?
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38.5 What does the whole system look like?
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38.6 How does the WebSocket gateway work?
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38.7 How do message IDs and ordering work?
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38.8 How do delivery and read ticks work?
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38.9 How does offline delivery work?
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38.10 How does group chat work?
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38.11 How does media work?
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38.12 What would you say in the last five minutes?
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Score yourself
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Variations to practise
16
Sources
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Code lab
38. Design a Chat App (WhatsApp)

Sources

3 min read+10 XP

Source notes

  • WhatsApp group size: raised to 512 and then to 1,024 participants in 2022, as reported at the time. Checked 2026-10-08.
  • End-to-end encryption with the Signal Protocol, and sender keys for groups: WhatsApp Encryption Overview (security whitepaper). Checked 2026-10-08.
  • Gateway sizing and reconnect behaviour: see Chapter 21.
  • All user, message and connection numbers are interview assumptions agreed in Step 1, not figures for any real app. Arithmetic computed 2026-10-08.
  • Routing, client-ID de-duplication, per-chat sequence numbers, multi-device ticks, cursor sync and group cost: tested 2026-10-08 (code/ch38_chat.py, all checks pass).